Significant progress has been made in the development of low cost, reliable electroactive fluids. Generally, electroactive fluids consist of suspensions of very fine particles in a dielectric liquid media. Electroactive fluids experience changes in their physical properties in the presence of an electric field, and for this reason are useful in a wide variety of mechanical treatments.
One type of electroactive fluid is an electrorheological or "electroviscous" fluid. For example, see Carlson, U.S. Pat. No. 4,772,407. Electrorheological fluids are electroactive fluids which, in the absence of an electric field, exhibit Newtonian flow characteristics such that their shear rate is approximately proportional to shear stress. However, when an electric field on the order of 10.sup.3 V/mm is applied, a yield/stress phenomenon occurs such that no shearing takes place until the shear stress exceeds a yield value which rises with increasing electric field strength. The result can appear as an increase in apparent viscosity of several orders of magnitude.
Another type of electroactive fluid is an electrophoretic or "electroseparatable" fluid. Electrophoretic fluids are described in Klass, et al., U.S. Pat. No. 3,255,853; and in Carlson, U.S. patent application Ser. No. 463,245, assigned to the assignee of the present invention. Electrophoretic fluids are suspensions similar to electrorheological fluids but are characterized by a very different response to an applied electric field. The particles within electrophoretic fluids exhibit a very strong electrophoretic migration. Rather than forming, in the presence of an electric field, a fibrillated structure that has an induced yield strength, electrophoretic fluids separate into particle-rich and particle deficient phases by electrophoresis. Generally, the electrophoretic induced separation may be accomplished and maintained at much lower electric fields, since electrophoresis is a linear phenomenon with respect to electric field strength; while in contrast, the strength of an electrorheological fluid varies with the square of the electric field because of the dependence on induced dipole interactions for the electrorheological effect.
Many commercially realizable systems employing electroactive fluids have been developed which include torque transmission and conversion devices such as variable clutch, brake and differential assemblies. See Winslow, U.S. Pat. No. 2,886,151 and Klass, et al., U.S. Pat. No. 3,255,853. Assemblies of this type are well known and offer the possibility of progressive and continuous control of torque in response to variation in the electric field applied to the electroactive fluid.
However, a common problem associated with electroactive fluid assemblies is containment of the electroactive fluid where a rotary shaft must pass through a seal into a region filled with the fluid. The leakage of fluid is difficult to prevent particularly for systems operating under conditions of prolonged vibration, impact, heat and under other adverse conditions. Conventional dynamic fluid seals are not well suited to this application since the particles (the particulate phase) of electroactive fluids may be rather course and abrasive. Temperature and pressure extremes experienced by the contained fluid, even under normal operating conditions, further may reduce the effectiveness of the seal causing fluid leakage. Loss of electroactive fluid through a dynamic fluid seal in such systems is especially undesirable since voids in the fluid containment region may cause electrical arcing or high voltage breakdown which adversely affect system performance and which also may lead to system failure. Further, leakage of the electroactive fluid liquid media (the liquid phase), but not the particulate phase, can have deleterious effects on the fluid rheology, resulting in sedimentation and caking of the particulate phase.
Rotary-shaft seals employing a ferrofluid in a magnetic circuit have been developed for inert gas or vacuum seal applications which are designed to withstand significant pressure differences under static, as well as dynamic, conditions.
Ferrofluids are permanent, colloidal suspensions of ferromagnetic particles such as magnetite or Fe.sub.3 O.sub.2 in various carrier solvents with added stabilizers which prevent agglomeration of the particles. Ferrofluids are described in the publication of R. Rosensweig, "Magnetic Fluids," Scientific American, Vol. 243, pp. 114-132, October 1982; and in Rosensweig, U.S. Pat. No. 3,917,538. Unlike the coarse magnetic particle suspensions used in the non-Newtonian magnetic clutch fluids of the 1940's, the particles of ferrofluids are sufficiently small (approximately 100 angstroms in diameter) that they are retained in suspension by Brownian motion forces. Further, because of their size, they do not exhibit a Bingham type yield stress behavior and congeal into a solid mass when a magnetic field is applied, but instead remain as a homogeneous liquid. When a magnetic field is applied to a ferrofluid, a body force is developed which is simply a force that acts throughout a given volume of the fluid. The body force developed depends on the gradient of the magnetic field and the magnetization value of the ferrofluid and causes it to occupy the region or regions of highest magnetic field.
A multistage, rotary-shaft ferrofluid seal is shown, for example, in Rosensweig, U.S. Pat. No. 3,620,584. The seal includes a magnetic circuit consisting of a permanent magnet and pole pieces to provide a focussed magnetic field. The ferrofluid thereby experiences a body force sufficient to entrain discrete, ferrofluid liquid O-rings in each annular stage of the seal.
However, ferrofluid seal assemblies have heretofore not been recommended for use in sealing liquids under pressure since many liquids tend to be reactive or miscible with the ferrofluid resulting in flocculation, leakage and other problems. Furthermore, ferrofluids are not well suited for use in sealing arrangements that involve particles in a packing mode since the particles will collect in the region of the seal so as to interfere with seal integrity.
In view of the foregoing, there is a need for an electroactive fluid torque transmission and conversion apparatus which successfully incorporates a ferrofluid seal for improved containment of the electroactive fluid.